Leaders of California's largest cities are backing legislation that would expand and improve California's film and TV tax credit program.

The mayors of Los Angeles, Sacramento, Long Beach, San Francisco, Fresno, San Diego, Bakersfield, Santa Ana, Oakland and San Jose on Tuesday signed a letter supporting Assembly Bill 1839.

"Extending California's film and television production tax credit program is a smart, prudent investment in California's future and economic competitiveness," the mayors wrote. "The program is one of California's most efficient and proven economic development tools, generating 51,000 jobs and providing $4.5 billion in direct spending since its inception in 2009."

The bill, reviewed today by an Assembly panel, would significantly expand funding and allow more projects to qualify so that California can better compete with film incentives offered by rival states and countries that have grabbed a large share of Hollywood's business.

In 2012, only one big budget feature film was shot entirely in California, and in 2013, just 39 out of 137 one-hour television series were filmed here, the mayors noted in their letter.

AB 1839 would replace a program enacted in 2009 that allocates $100 million annually to film and TV projects and that is set to expire in 2017. The program currently excludes many types of productions, including large studio films and new network dramas that increasingly are being filmed outside of the state.

"The widespread geographic and bipartisan support for taking this action demonstrates a keen realization that California is losing tens of thousands of middle class jobs and significant tax revenue to other states and nations when it comes to film and television production," the mayors wrote.

Their letter was addressed to Assemblymen Mike Gatto (D-Los Angeles) and Raul Bocanegra (D-Pacoima), who chairs the Assembly Revenue and Taxation Committee, where the AB 1839 is expected to be approved Tuesday before going to the Assembly for a full vote this summer.

The bill previously was unanimously approved by the Assembly Arts, Entertainment, Sports, Tourism and Internet Media Committee in March.

The legislation is expected to face tougher road in the Senate, however, where some Northern California lawmakers have opposed offering subsidies to the film industry.

A recent report from the Legislative Analyst's Office urged lawmakers to be cautious about expanding the program, saying the fiscal benefits had been exaggerated. Also uncertain is whether Gov. Jerry Brown will endorse the measure.

In response, a coalition of labor, industry and government officials are stepping up lobbying efforts. The mayors' letter is the latest in that effort.

"In order to once again be competitive, California must put in place a meaningful, expanded credit that will bring back jobs, increase revenue, and support small businesses and vendors all across the state. Too much is at stake for the people of California to let this key industry slip away," the mayors added.